Many factors are known that can contribute to and/or exacerbate an occlusion of a bodily vessel or lumen. Such factors include internally induced vascular injury, such as, for example, vascular injury caused by an accumulation of plaque at the walls of the lumen, or externally induced vascular injury, such as vascular injury caused by the deployment of a stent and/or by angioplasty. When a lumen is subjected to injury, white blood cells and other substances tend to converge on the injured region of the lumen bringing about inflammatory effects at that region. These inflammatory effects tend to result in an occlusion of the lumen, that is, an undesired narrowing of the lumen or a total blockage of the lumen. In the case of an externally induced injury resulting from a deployment of a stent, the inflammatory effects sometimes tend to contribute to what is termed restenosis, or re-occlusion of the lumen.
Moreover, in the lumen, there may be areas with flow disturbances. The vasculature in the human body is a tree of diverging lumens with typical dichotomous diverging patterns. The dichotomous diverging patterns create bifurcations in the arterial system where the mother lumen diverges into two daughter lumens. In most cases one of the daughter branches is larger and is the continuation of the mother vessel and the other one is smaller and the side branch. (The venous system is basically the mirror image of these bifurcations in which the flow is from smaller lumens into larger ones, or in the direction of flow one can describe this system as converging rather than diverging.) The present invention is in part based on an observation that vascular injury may be caused by or exacerbated by among other things flow disturbances in the flow of blood at a given region of the lumen. These flow disturbances include areas of “flow separation,” in which vortices are created next to the vessel wall, rendering the blood essentially stagnant at those locations. Such areas of flow separation are a particular problem in bifurcated lesions. However, even lumens with a substantially constant diameter may have areas with undesirable flow separation or other flow disturbances.
Such disturbances may work to increase the likelihood of inflammation by increasing the residence time for inflammatory cells near the walls of the lumen, and decreasing of flow forces that might otherwise push those cells downstream. That, in turn, may increase the probability that such white cells will penetrate the wall and will initiate an inflammatory effect, which can result in an undesirable narrowing of the lumen. Flow disturbance may also be an instigator in endothelial cell dysfunction (whether due to decreased fluid shear stress at the wall, imposition of bi-directional shear stress, or otherwise) which then promotes the occurrence of an inflammatory reaction. Finally, flow disturbance may cause or exacerbate vascular injury by promoting particle sedimentation and by inhibiting proper transport of waste materials from the vessel wall into the lumen. Although the precise mechanism or mechanisms by which flow disturbance accelerates the growth of vascular lesions is not completely understood, it is believed that avoiding flow disturbance will lessen the risk of vessels narrowing or re-narrowing.
Moreover, a typical angioplasty procedure causes trauma to the vessel wall. The angioplasty procedure is typically at the area of highest constriction in a vessel, i.e., at the location of a lesion. If a stent is implanted in this area, flow disturbances may be present.
Without limiting the scope of the present invention, there are at least three factors that may influence the presence of flow disturbances at areas of constriction. First, it is unlikely that the lesion formed by chance—lesions rarely do. There must be a reason why the lesion is localized at a particular place. More than likely, this place was a point of flow disturbance to begin with. At best, stents according to the prior art do not alter the geometry of a vessel but rather return it to the original geometry. Hence there is a good chance that the stented geometry includes the flow disturbance.
Thus, an angioplasty or stent procedure likely increase inflammatory effects. First, as described above, any flow disturbances may increase the residence times for inflammatory cells near the walls of the vessel even in the absence of injury, and/or adversely effect endothelial cell function. Second, the vascular injury caused by the angioplasty and/or stent deployment enhances the likelihood of inflammation, even in the absence of flow disturbance.
Furthermore, as opposed to the optimal stent deployment discussed above, stents often do not deploy optimally, thus introducing a new flow disturbance into the system. This can be because they are not adequately tapered (which is a particular problem with conventional dedicated bifurcation stents), or because they are deployed in a curving vessel and alter that curvature, or because of sub-optimal support of a tough lesion or just because of the nature of the stent design, or because most stents (and particularly those that are dedicated bifurcation stents) do not have an angle of taper.
There are known stents which are tapered. For example, U.S. Pat. No. 5,827,321 to Roubin et al. discloses a tapered stent. The stents disclosed in that patent, however, are tapered to provide an optimized fit between the original geometry and the treated section of the lumen, so as to minimize the stretch of the lumen wall when treating a tapered section of a lumen. They are therefore specifically designed not to alter the flow characteristics of a lumen. Rather, they are specifically designed to conform to the existing geometry of the lumen.
In view of this there is a need to minimize flow disturbance and the concomitant probability of inflammatory cell adhesion/infiltration where adhesion/infiltration is deemed to be a problem. In particular there is a need to provide a method an apparatus for reshaping a lumen to promote laminar flow and minimize or eliminate any areas of flow separation, especially in areas that were traumatized by the angioplasty procedure, in order to minimize the effect of the trauma. This includes assuring the proper shaping of the lumen when stenting a bifurcation or even to creating an artificial geometry in a straight vessel where such shape was non-existing prior to the treatment, in order to minimize flow separation, that may happen if there is no shaping. Preferably, this can be done by physicians using existing devices that were not originally made to create an artificial geometry